The specific problem is the elucidation of the metabolic transformation of porphobilinogen in uroporphyrin III and uroporphyrin I. A large group of diseases known as porphyrias can be related to this transformation. In this diseases, the patients suffer from a derrangement in the heme metabolism which leads to a massive accumulation in their body of porphobilinogen and uroporphyrins, specially the pathological uroporphyrin I. These diseases are congenital (leading to an early death during puberty), or acquired through polluting agents such as lead or plaguicids. We propose to study how porphobilinogen is transformed into the normal uroporphyrin III, and how the level of porphobilinogen is regulated in the metabolism. We think that a group of mixed-function oxidases which we discovered and called pyrrolooxygenases is responsible for this regulation acting by both oxidizing porphobilinogen to a biological harmless derivative, and by inhibiting the enzymes involved in uroporphyrin formation. Since no efficient cure is known for porphyrias, the regulation by these oxygenases may represent a useful approach to the control of the disease.